Cyclones or tube separators are known from the state of the art for separating liquids from gases, in the case of which the throughflowing air is set in a rotary motion and thus leads to separation of the particles due to centrifugal forces. Tube separators of this type are incorporated for example in cylinder head covers in order to clean there the blowby gases from the crankcase of an internal combustion engine of oil and oil mist. The pressure side of the tube separator is thereby connected to the crankcase, whilst the suction side guides the gases cleaned of oil or oil mist to the inlet manifold of the engine. The separated oil is generally guided back into the camshaft housing via a siphon. The siphon thereby has two functions. On the one hand, it leads to drainage of the oil from the suction-side chamber of the oil separator into the camshaft housing which belongs to the pressure side and, on the other hand, it leads to a barrier so that a pressure difference between the pressure side of the oil separator and the suction side of the oil separator can be maintained. Furthermore, the barrier effectively prevents oil spurting from the camshaft for example passing into the suction region.
A siphon of this type is illustrated in FIG. 1, the partial Figures A and B representing two different embodiments. A siphon 1 has accordingly two pipes 2 and 3 which communicate with each other at their lower end. The top pipe 2 is thereby connected by its upper side to the chamber to be drained, in the case of the oil separator in an internal combustion engine to the suction-side chamber of the oil separator. The outlet pipe 3 communicating with the top pipe 2 is open at the top and forms an overflow opening into the camshaft housing. Between the inlet 4 of the top pipe 2 and the outlet 12 with the outlet edge 7 of the outlet pipe 3 there is a height difference h2 which corresponds to the maximum pressure difference between the inlet side and the outlet side of a barrier siphon. This maximum barrier differential pressure in the case of the prevailing drainage is Δp=h2*ρ, ρ being the density of the liquid situated in the siphon (e.g. 0.9 for mineral oil at room temperature or 1.0 for water at room temperature).
If no drainage takes place and no oil ejected from the camshaft housing gets into the siphon, i.e. no liquid is situated in the volume V1 of the outlet pipe which is situated above the edge 6 between the top pipe 2 and the outlet pipe 3, then the maximum barrier pressure difference without drainage is Δp=h*ρ=(h1+h2)*ρ. h is thereby the length of the top pipe between its upper edge 5 and the lower edge 6 and h1 is the length of the outlet pipe 3 between the lower edge 6 and its upper edge 7. The barrier height is reduced in case the volume V3 of the outlet pipe 3 is smaller than the product of h and the average cross section A2 of the top pipe 2. The barrier height is then referred to as effective barrier height with heff=V1/A2+h1.
FIG. 1 now represents two different siphon types, the top pipe 2 in FIG. 1A and the outlet pipe 3 having a common wall, the lower end of which forms the edge 6.
In FIG. 1B the inlet 4 is situated laterally in the top pipe 2, whilst the outlet pipe 3 is disposed within the top pipe 2. The outlet 12 is situated here in the centre. In this embodiment, both the top pipe 2 and the outlet pipe 3 have an annular cross-section.
If now a separated liquid flows via the inlet 4 into the top pipe 2, in the case of drainage, then it passes under the edge 6 into the outlet pipe 3. In the case of sufficient inflow of liquid, the liquid level rises in the volume V1 in the outlet pipe 3 until the liquid overflows over the outlet edge 7.
In the case of oil separators for blowby gases, a differential pressure exists between inlet 4 and outlet 12 which corresponds to the pressure drop across the oil separator, not shown here. If the pressure drop and hence the differential pressure between the outlet 12 and the inlet 4 increases, then liquid is suctioned into the top pipe until the height difference between the liquid level in the top pipe 2 and the liquid level in the outlet pipe 3 corresponds to the pressure difference. If the low pressure on the inlet side 4 increases further, then the result can be that the liquid situated in total in the volume V1 is suctioned into the top pipe 2 so that finally gases are suctioned via the outlet pipe 3 into the top pipe 2. At this moment, the barrier effect of the siphon is removed and gases are suctioned towards the inlet 4 through the top pipe 2. Furthermore, the latter entrain the liquid situated in the top pipe 2 also. Hence drainage of fluid is no longer provided from the inlet 4 to the outlet 12.
Situations of this type can occur, for example if the resistance in the oil separator which is present increases due to uncontrolled or excessive increase in gas volume flow and hence, between the inlet manifold (inlet 4) and the crankcase (outlet 12), a very high differential pressure is formed. In this case, the oil collected in the volume V1 is drawn back into the valve cover and the siphon opening as a result can be subjected subsequently to a flow of blowby gases laden with oil particles. The oil which is suctioned upwards will then discharge in the direction of the inlet manifold and can lead to damage to the engine. This so-called oil entrainment is greatly feared by engine manufacturers. When using a tank with a valve, as is common in the state of the art, the crankcase can also be pressurised in this operating state, which leads to lack of sealing at the corresponding bearing seals. Tank solutions are therefore equipped with an excess pressure valve on the oil separator.
It is however disadvantageous in this respect that this excess pressure valve has mechanical elements as bypass circuit and, for its part, can become soiled. As a result, it can become permanently unsealed or also completely clogged. On a long term basis no reliable function of these bypass circuits is hence ensured.